This project has four major objectives: 1) to elucidate the nonvisual molecular function of vitamin A, 2) to examine the metabolism of carotenoids and vitamin A in relation to their biological activity, 3) to determine the mechanisms by which vitamin A is transferred into epithelial cells, across the placenta and into milk, and 4) to develop more suitable procedures for evaluating total body stores of vitamin A in experimental animals and in human beings. With respect to the nonvisual function of vitamin A, three major approaches will be taken: 1) to correlate changes in epithelial tissue structure and metabolism in rapidly induced vitamin A deficiency, 2) to isolate specific receptors for vitamin A within target cells, and 3) to evaluate the relationship of vitamin A to mucus secretion in selected glands. With respect to metabolic transformations, the conversion of astaxanthin to retine in fish and crustacea, and the relationship between the metabolism and biological active of 15 methyl retinol will be studied. With respect to the transport of vitamin A, the kinetics, form and mechanism by which vitamin A is taken up from the plasma by epithelial cells and by milk-secreting cells of the mammary gland will be examined, and the manner in which vitamin A crosses, or is restricted from crossing, the placenta will be investigated. Finally, attempts will be made to devise new methods for evaluating total body stores of vitamin A based on 1) excretion patterns of metabolites, 2) isotope equilibrium with labeled, well-retained derivatives of vitamin A, and 3) the % saturation of retinol binding protein.